Polar Bismuth Selenite Iodate Oxide BiSeIO₆ with Three Types of Lone Pair Cations in One Structure

Novel bismuth selenite iodate oxide BiSeIO₆ was synthesized in a mild hydrothermal condition. BiSeIO₆ was crystallized in the polar space group Pna2₁ of an orthorhombic system. The crystal structure features a three-dimensional framework composed of three types of lone pair cations with distorted BiO₇ polyhedra, SeO₃ pyramids, and IO₃ pyramids in one structure. Interestingly, BiSeIO₆ exhibits a strong and phase-matchable second-harmonic generation (SHG) of ∼6 times that of KH₂PO₄ (KDP). Dipole moment analysis shows that all three local acentric groups of BiO₇, SeO₃, and IO₃ cooperatively contribute to the large macroscopic polarization and thereby strong SHG efficiency of BiSeIO₆. In addition, BiSeIO₆ has a broad transparency range from 0.35 to 11 μm, indicating its promising nonlinear optical applications from visible to mid-infrared bands.